FileIconProvider: Do some more caching ourselves

QFileIconProvider already does some pixmap caching, but only in some
cases (non-executable, non-link). Since QFileIconProvider is expensive,
we want as much caching as possible.
It still should be investigated why it seems to be even more expensive
on OS X 10.9 in some cases though.

Task-number: QTCREATORBUG-10491
Change-Id: I5a0f995eafdced47dc6da869c72a1ca016b25f63
Reviewed-by: Daniel Teske <daniel.teske@digia.com>